Add variadic sugar for boolean static methods.

[fortmarek]

Hi 👋

This is my first contribution, albeit a small one.

What I'd like to add is variadic argument support for static intializers of Signals, SignalProducers and Properties where Value == Boolean.
So, similar to how we write SignalProducer.combineLatest(producer1, producer2) we could write SignalProducer.any(producer1, producer2) - right now we have to do SignalProducer.any([producer1, producer2]) which seems unnecessarily verbose.

I have also change a few test cases to leverage this new feature

Checklist

• Updated CHANGELOG.md.

[RuiAAPeres]

I am slightly on the fence about this introduction, since the implementation, as-is is quite shallow. I will defer this to @andersio.

[andersio]

@RuiAAPeres I think it's alright, basically sugar over the existing variant taking a Collection. Doesn't introduce new logic, and doesn't introduce overload conflicts.